The latter half of the twentieth century has been witness to a phenomenon known as the information revolution. While the information revolution is a historical development broader in scope than any one event or machine, no single device has come to represent the information revolution more than the digital electronic computer. The development of computer systems has surely been a revolution. Each year, computer systems grow faster, store more data, and provide more applications to their users.
The extensive data storage needs of modem computer systems require large capacity mass data storage devices. While various data storage technologies are available, the rotating magnetic rigid disk drive has become by far the most ubiquitous. Such a disk drive data storage device is an extremely complex piece of machinery, containing precision mechanical parts, ultra-smooth disk surfaces, high-density magnetically encoded data, and sophisticated electronics for encoding/decoding data, and controlling drive operation. Each disk drive is therefore a miniature world unto itself, containing multiple systems and subsystem, each one of which is needed for proper drive operation. Despite this complexity, rotating magnetic disk drives have a proven record of capacity, performance and cost which make them the storage device of choice for a large variety of applications.
A disk drive typically contains one or more disks attached to a common rotating hub or spindle. Each disk is a thin, flat member having a central aperture for the spindle. Data is recorded on the flat surfaces of the disk, usually on both sides. A transducing head is positioned adjacent the surface of the spinning disk to read and write data. Increased density of data written on the disk surface requires that the transducer be positioned very close to the surface. Ideally, the disk surface is both very flat and very smooth. Any surface roughness or “waviness” (deviation in the surface profile from an ideal plane) decrease the ability of the transducing heads to maintain an ideal distance from the recording media, and consequently decrease the density at which data can be stored on the disk.
The disk is manufactured of a non-magnetic base (substrate), which is coated with a magnetic coating for recording data on the recording surfaces, and which may contain additional layers as well, such as a protective outer coating. Historically, aluminum has been the material of choice for the substrate. As design specifications have become more demanding, it is increasingly difficult to meet them using aluminum, and in recent years there has been considerable interest in other materials, specifically glass. Glass or ceramic materials are potentially superior to aluminum in several respects, and offer the potential to meet higher design specifications of the future.
One of the major drawbacks to the use of glass or ceramic disk substrates is the cost of their manufacture. Glass is currently used in some commercial disk drive designs, although generally at a higher cost than conventional aluminum. In a typical glass disk manufacturing process, the glass base material is initially formed in thin glass sheets. Multiple glass disks are then cut from a sheet. The circumferential edges are finished, which typically requires multiple process steps. The broad, flat data recording surfaces are then lapped to reduce waviness, and polished to a smooth finish, which again may require multiple process steps. The glass substrate is then subjected to a chemical strengthening process, in which the disks are immersed in a salt bath and an ion exchange takes place between ions in the glass and ions in the salt. The glass substrate thus formed and strengthened is then coated with a magnetic recording layer, and may be coated with other layers such as a protective layer. Additionally, there are typically multiple intermediate cleaning steps for cleaning off process residues.
Related to process cost is the disk yield from the manufacturing process. Disk requirements have very demanding tolerances, and many of the processes have the capability to introduce defects into the disks. Glass disk manufacture being a relatively new field in comparison to manufacture of older aluminum-based disks, yields from glass disk manufacturing processes are often not as good as is typical of older, more established processes. Continued yield increases require continued improvements to the various manufacturing process steps to which a glass disk is subjected.
Finally, related to both cost and yield is the issue of overall disk quality. Some disks, although passing manufacturing inspections or meeting minimum specifications, nevertheless fail in the field or otherwise do not perform as well as expected due to quality defects that were not anticipated by those who designed the manufacturing inspection procedures or generated the disk specification requirements.
An understanding of the effect of individual manufacturing process steps on overall cost, yield and quality can lead to improved manufacturing approaches. Often, it is this understanding, not obvious in itself, which suggests a process improvement. In some cases, the process improvement is relatively simple, given the understanding of the cause of a recurrent process deficiency.
Unless the cost, yield, and quality of glass disk manufacturing processes can be substantially improved, it will be difficult to replace aluminum with glass and realize the potential benefits that glass disks offer.